182 RevieiDS — Br. A. Bigot — Archcean and Cccmbrian of France. 



Dufrenoy's identification was accepted by Dalimier, Bonissent, De 

 Lapparent, and finally by Barrois, though it should be remarked 

 that the last-named writer has been wont to quote Dufrenoy's 

 opinion without positively endorsing it. Professor Ilebert, however, 

 adopting a Cambrian age for the "Conglcmerats pourpres," and 

 observing that these strata were unconformably superimposed upon 

 the " Schistes de Saint-L6," concluded that the latter were of 

 Archaean age. In this opinion, the late Professor of Geology at the 

 Sorbonne has been followed by his pupil, Dr. Bigot. 



The evidence given in the volume before us would seem to be 

 decisive of the true relation of the Saint-L6 Series to the purple 

 conglomerates. The author confirms the existence of the strati- 

 graphical discordance alleged by Hebert, and he affirms that the 

 newer series contains pebbles of the sandstone which is intercalated 

 amongst the Phyllads. The unconformity would therefore seem to 

 be very marked. 



The researches of Dr. Ch. Barrois and Dr. Bigot have brought to 

 ligbt additional resemblances between the rocks of Saint-L6 and the 

 younger Archgeans of Britain. According to the former writer, the 

 Saint-L6 Series sometimes presents a volcanic facies. In the north 

 of Brittan}^ acid rocks, such as " quartz-porphyries and petrosilex," 

 are said by him to be interstratified with the Phyllads, and to 

 furnish rounded fragments to the Conglomerates of Montfort, the 

 equivalents of the " Conglomerats pourpi-es." Dr. Bigot records 

 similar facts. He states that, at La Hogue and in the island of 

 Alderney, the " purple conglomerates " contain pebbles of *' petro- 

 siliceous porphyries," evidently derived from an older formation. 



According to Dr. Bigot, the district of La Hogue furnishes still 

 more emphatic proof of the gap between the Phyllads and the 

 purple conglomerate. A quarry displays a section of the Phyllads, 

 penetrated by numerous veins of " granulite," and surmounted by 

 the purple series. The veins are described as modifying the 

 Phyllads, but producing no effect upon the purple beds ; and it is 

 inferred by Dr. Bigot that the older group was invaded and meta- 

 morphosed by the granite previous to the deposition of the purple 

 conglomerates. 



We are gradually learning that the resemblances between the newer 

 Arch^an rocks in Northern France and Western Britain are very 

 close indeed. In the Bi'itish area, the predominating types of rock 

 in the Pebidian (or Uriconian) are acidic volcanic eruptives, hypo- 

 metamorphic schists, grits more or less altered, and comparatively 

 unaltered slates ; and these are precisely the lithological characters 

 most conspicuous in the Phyllads of Saint-L6. The present writer 

 accompanied Dr. Barrois and Dr. Bigot in the year 1888 to Caer 

 (Jaradoc. Both of these geologists were struck with the lithological 

 similarities between our Uriconian and the volcanic rocks associated 

 with the Phyllads, and they have recorded their impressions in 

 subsequent publications. 



But the parallelism between the older rock-groups of the French 

 and British areas has been shown to extend yet further. Dalimier, 



